Without limiting the scope of the invention, its background is described in connection with heterojunction bipolar transistors (HBTs), as an example.
Heretofore, in this field, heterojunction bipolar transistors have suffered from performance degradation and catastrophic failure when operated under high power conditions. The cause of these problems has generally been attributed to that of a condition of positive feedback between transistor current and either the base-emitter or base-collector junction temperature. High power HBTs typically have multiple emitter fingers so that the transistor can handle higher currents. The problem of positive feedback occurs when localized heating occurs on one of the emitter fingers, forming a "hot spot". As the junction temperature rises in the vicinity of the hot spot, the collector current increases in the finger. The larger current causes the junction temperature to rise further, thereby inducing more current to flow. Eventually, the total current in the multi-finger transistor attempts to flow through the single finger, thus leading to thermal runaway and a catastrophic failure. Past solutions to this problem have relied on a resistor in series with each emitter finger having a positive relation of resistance to temperature. Therefore, as the current through the finger increases with rising junction temperature, the same increased current flow through the emitter, or "ballast", resistor causes an increase in resistance that limits the emitter-collector current through the emitter finger. This technique, the use of ballast resistors external to the transistor, has been in use for many years. See S.M. Sze, Physics of Semiconductor Devices, 2nd Edition, p 169. Only with the recent development of deposition techniques such as Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD), has it become possible to incorporate the ballast resistor into the transistor structure. Emitter ballast layers of lightly doped GaAs have been used in HBTs, see Gao, et. al, IEEE Transactions on Electron Devices, vol. 38, 2, pp. 185-196, February 1991.
In addition to the catastrophic "collapse" phenomenon, a negative differential resistance (NDR) is observed in the current-voltage curves of typical HBTs operated at moderate power levels where junction temperature is increased. However, unlike "collapse", NDR is characterized by a gradual decrease in current gain as the power level increases. This NDR problem can be seen in a single emitter finger transistor, whereas the collapse phenomenon is usually seen in multiple finger devices. The cause of the NDR characteristic is thought to be that of decreasing emitter injection efficiency as the junction temperature increases. The problems of collapse and NDR seem to be related to the design of the emitter of the HBT, therefore an emitter design that overcomes these problems is desirable.